This invention relates to dental implants and, more particularly to blade type implants having posts without necks.
A dental implant, such as that described in U.S. Pat. Nos. 3,465,441 and 3,660,899 of the present inventor, is used to support an artificial bridge, tooth or other dental prosthesis. The implant has an implant portion, e.g. in the form of a blade, that is secured in the underlying bone in an edentulous span. A post portion, typically with a narrowed neck part, extends up from the implant portion and supports the artificial bridge or crown. This type implant is inserted by making an incision in the fibromucosal tissue down to the underlying alveolar ridge crest bone. The tissue is then reflected to expose the bone and a burr is used to create a groove in the bone which is as deep as the implant portion The implant portion is then wedged into the bone. After the insertion, the tissue is sutured about the neck part so that the rest of the post protrudes above the tissue line. Typically, a few weeks or months are allowed to pass before the dental prosthesis is attached to the post. During this period, bone starts to grow around the implant portion and through holes provided in it, thereby acting to anchor the implant in place before it is stressed by use.
Submergible blade implants, such as that shown in U.S. Pat. No. 4,177,562 of A. L. Miller and A. J. Viscido, allow a blade to be inserted in the jawbone for a long period of time before being placed in actual use. With this type of implant the blade is completely submerged in the bone. It is then covered over with tissue and allowed to remain in place for several months. For this period it is protected against being dislodged by the tongue or other teeth during mastication. Once there has been substantial regrowth of the bone over, around and through the submerged blade, the tissue is again opened and the post is attached to the blade, typically by a screw connection. In order to accommodate the screw, however, it is necessary for the implant body or blade to be relatively wide, at least in the area of the screw. This limits the use of the implant in patients with narrow ridge crest bones. In addition a collar is frequently provided at the location of the screw, which collar projects out of the bone and the gum tissue as in the design disclosed in the Miller patent mentioned above. As a result of this collar bacteria can invade the tissue around the implant, which may lead to an infection that causes the implant to fail.
When a semi-submerged implant is used, i.e. an implant wherein a screw collar projects out of the bone, the collar is sometimes used as an abutment to support overlying provisional dentures. Such use can easily loosen the implants during the healing stages, since only one or two of these collars act as the sole support of the denture. Also, even if the provisional dentures are hollowed out so as to be larger in diameter than the collars and to thereby avoid contact with them, food and other debris can become lodged in this space created under the dentures. The decay of this food can cause tissue breakdown with subsequent implant failure.
As noted, it is common for many types of prior oral implants to have a post with a neck portion which connects to a blade. Such a neck portion is typically much narrower than the rest of the post and the blade. In view of that, a step-type transitioned area is defined between the post and the blade. The steep variations between the dimensions of the blade, post and the neck make the transitioned area subject to a much greater concentration of the stresses of mastication than other areas of the implant. All this makes the design of the narrow neck the weak spot of the oral implants disclosed by the prior art. In use, such implants can bend in the area of the neck portion when chewing movements are performed. This can cause bone resorption immediately below the neck portion and breakage of the neck.
U.S. Pat. No. 4,178,686 to Riess et al. provides an oral implant in which the implant portion is a polymer matrix having spherical particles of tricalciumphosphate ceramic embedded in its exterior. A post portion has an elongated core member extending substantially into the implant portion. The base of the top part of the post portion extends to the outer edges of the implant portion and tapers inward in the part towards the artificial tooth support. The tooth support itself may be attached to the post by means of a threaded shaft. With this arrangement, the forces of mastication are resisted solely by the threaded shaft, which is relatively narrow, and/or the narrow core portion of the post.
The present inventor's own U.S. Pat. No. 4,600,388 discloses a blade in which the post is designed to straddle recessed portions in the blade. Because of these recessed portions in the blade, the post does not extend beyond the outer limits of the blade. Further, the legs of the implant, that allow it to straddle the blade, are relatively thin and these thin legs must resist the forces of mastication. With this implant there is no direct means for rigidly securing the post to the blade, other than the natural spring force of the legs of the post.
Sometimes the major portion of bone is such that an implant located in the middle of the bone would position an artificial tooth at an angle which would cause it to be out of alignment with other teeth To solve this problem, various devices are proposed in the present applicant's U.S. Pat. No. 4,713,004.
The relatively narrow neck portions of posts in prior art implants are subject to bending and breakage during normal use. When this occurs it is often necessary to remove part or all of the implant, including the blade portion, to repair the damage. Thus, it would be extremely advantageous if blade implants could be provided with extremely rugged post portions which could easily resist the forces of mastication.